Our objective is to understand how embryonic cells detect their differing positions with respect to other embryonic landmarks, and how positional cues act on cells to bring about their commitment to specific developmental pathways. This problem is here addressed in the embryo of the leech, but by providing insight into the factors which govern normal and abnormal development the proposed research will also contribute to our understanding of human birth defects. The o and p bandlets of the leech embryo are parallel columns of ectodermal blast cells which arise from the fertilized egg via stereotyped cell lineages. Although the blast cells in these two bandlets are born with the same developmental potential, they subsequently interact with one another in a position-specific manner and diverge into distinct O and P developmental pathways. One major aim of this study is to characterize the means by which the blast cells interact, and to identify the cellular and molecular cues which allow them to detect their relative positions. The proposed experiments will examine whether o and p blast cells must be in direct contact for the interaction to occur, and whether their interaction is mediated by the gap junctions that link them. The role of gap junctions will be ascertained by injecting blast cells with antibodies against junctional proteins that have been shown, in some systems, to be capable of silencing transjunctional communication. The other major aim of this study is to learn how the positional cues commit an o or p blast cell to only one of the two pathways that it is initially competent to follow. Commitment has been shown to occur in a series of discrete steps, and the cellular basis of this stepwise commitment process will be investigated further. Particular attention will be given to the hypothesis that each step in the commitment process is associated with a particular blast cell division. Commitment entails a loss of interaction between cells in the o and p bandlets, and additional experiments will investigate why it is that the committed cell ceases to interact with neighboring cells in the other bandlet.